This invention relates to pipe swivel joints, and more particularly to such swivel joints that are designed to withstand large external loads such as are encountered when used in the oil and gas industry.
A major advance in the completion of offshore wells in water depths at which diver assistance is uneconomical or otherwise impractical has been the design and development of diverless lay-away flowline type subsea Christmas trees for installation on subsea wellheads. These trees utilize the flexibility of the flowline to make the flowline-to-tree connection at the surface prior to lowering the tree to the subsea wellhead, thereby eliminating the need to make that connection at a subsea location and also permitting pressure testing the connection in the drillship moonpool. Prior to the present invention the flowlines of a lay-away system were connected to the tree in a fixed manner and extended from the tree at an angle, usually about twenty degrees from the vertical, which required more space in the moonpool than was always available, especially on drillships designed for guidelineless drilling and subsea completion systems.
One proposed solution to the problem involved connecting the lay-away flowlines to the universal guidebase by swivel joints, allowing the flowlines to hang vertically in the moonpool and then seek their final angle when they and the tree are landed on the sea floor. However, in this arrangement the flowlines cannot be run with the tree, whereby the highly desireable advantage of making up the flowline-to-tree connection at the surface is not available. Furthermore, this arrangement does not include the advantage of a flowline jettison feature, and the swivel joints employed at the guide base do not have metal-to-metal seals, a feature both highly desireable and frequently required by oil and gas industry specifications where elastomer or other non-metallic seals are inadequate.